Method for correcting position measurements for optical errors and method for determining mask writer errors

ABSTRACT

A method is disclosed for correcting errors introduced by optical distortions or aberrations in the measured values of coordinates of targets of an array of targets, like for example structures on a wafer or a photolithography mask. The array of targets is placed into a field of view of an imaging system via which the coordinates are to be measured. The array of targets is placed at different relative positions with respect to the field of view, and for each relative position the coordinates of the targets relative to the array of targets are determined by measurements. From the results of these measurements an alignment function, giving a correction for optical errors as a function of the position in the field of view, is derived. The measured coordinates are corrected by the alignment function. The corrected coordinates can be used to identify registration errors of a mask writer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Patent ApplicationSerial No. PCT/US2014/034391, filed on Apr. 16, 2014, which applicationclaims priority of U.S. Provisional Patent Application No. 61/812,377,filed on Apr. 16, 2013, which applications are incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

The present invention refers to methods for correcting positionmeasurements of targets, in particular to methods for correcting errorsin position measurements that are due to optical distortions across thefield of view of an imaging system used in a position measurementsystem. The present invention also relates to methods usingcorrespondingly corrected position measurements to identify errors inthe manufacture of photolithography masks.

BACKGROUND OF THE INVENTION

Position measurements of targets, in particular of structures onsubstrates in semiconductor manufacturing, are subject to various typesof errors. Precise determination of the position of structures isimportant to assure that, ultimately, correctly functioningsemiconductor products, like computer chips, for example, are produced.The demands with respect to precision increase as the structuredimensions on the chips to be produced decrease.

An important aspect of position measurements in the above context isregistration of structures or sections of a structured surface withrespect to each other. Errors of registration on a mask as determined bya measurement with a typical optical metrology tool, of whichKEA-Tencor's EMS IPRO 5 is a contemporary example, may for example bedue to errors in the optical metrology tool or to errors in the maskwriter. By eliminating or reducing the errors occurring in themeasurement with the optical metrology tool, the errors due to the maskwriter can be identified.

For example, German Patent Application Publication No. DE 10 2008 060293 A1 and United States Patent Application Publication No. US2011/0229010 A1 disclose a method for determining relative positioningerrors of plural sections of structures written on a substrate like awafer or a photolithography mask. One magnified image of a region of thesubstrate larger than one section is recorded. Position errors ofmeasurement marks contained in the image are determined from the image.The position errors are corrected for errors due to the imaging process.From the position errors corrected in this way the relative positionerror of the section is derived. This relative position error of asection is also known as stitching error, and the method assumes thaterrors due to the imaging process produce low frequency errors, whereasthe stitching errors produce high frequency errors. Therefore, in orderto remove the imaging errors, the low frequency error components areremoved by a high-pass filtering process.

Another approach is to measure each target in an array of targets, forexample each structure of interest in an arrangement of structures on ansurface of a semiconductor substrate, individually, by moving therespective target into the center of the field of view of an imagingsystem of an optical metrology tool and performing the measurement.

The multi-region-of-interest registration measurement is a furtherapproach. This makes use of the fact that often many targets aresimultaneously contained in the field of view of an imaging system of anoptical metrology tool. So the positions of plural targets, located atdifferent positions relative to the field of view, can be measured atthe same time.

However, the assumption made in the prior art about the mask writerhaving only high frequency errors is not strictly correct. By thehigh-pass filtering process information on the low frequency mask writererror therefore is discarded. In the case of individual targetmeasurements, the throughput is very low. For example, on an IPRO4metrology tool, measuring a single target may take up to 12 seconds, andmeasuring a typical array then up to 7 hours. During this long period oftime, drift errors of the metrology tool can occur, which reduce theprecision of the results.

In the multi-region-of-interest approach, due to optical distortion andaberrations which depend on the position in the field of view, differentregistration results may be produced if an array of targets like a maskwith structures is shifted relative to the field of view and theposition, relative to the array, or mask, respectively, coordinatesystem is determined for each shifted position. This error, depending onthe field-of-view coordinates and also referred to as field-varyingerror, limits the achievable precision of registration measurements.

The optical error, like for example the optical distortion and/oraberrations, depends on the optical setup of the imaging system, but mayalso depend on parameters of the measured targets/structures, like sizeor symmetry of the targets or on proximity of two or more targets. Theoptical error can further depend on the substrate on which an array oftargets is provided in specific technical fields, like in the case ofwafers or masks in semiconductor manufacturing. There, the optical errorcan for example depend on the coatings, layer design or layer thicknessof a mask.

BRIEF SUMMARY OF THE INVENTION

It therefore is an object of the invention to provide a method forcorrecting optical errors occurring in position measurements of targetsin a field of view of an imaging system, wherein the measurement can becarried out fast, variably and reliably.

This object is achieved by a method comprising the following steps:

-   -   a) placing an array of targets in a field of view of the imaging        system, wherein a plurality of targets of the array of targets        are within the field of view of the imaging system and wherein        the relative positions of targets within the array of targets        are fixed;    -   b) measuring the coordinates of the plurality of the targets of        the array of targets repeatedly via the imaging system, wherein        the array of targets is shifted relative to the field of view of        the imaging system between the repeated measurements;    -   c) determining an alignment function from the measurement        results of step b, the alignment function being a function of        coordinates of the field of view of the imaging system and        giving an additive correction for optical errors of the        coordinates of positions of targets measured by the imaging        system; and,    -   d) correcting the coordinates of the positions of the targets        measured by the imaging system by adding the respective value of        the alignment function at the field-of-view coordinates at which        the coordinates of the position of the respective target were        measured.

It is a further object of the invention to provide a method fordetecting photolithography mask writer errors wherein the measurementcan be carried out fast, variably and reliably.

This object is achieved by a method comprising the following steps:

-   -   a) placing a mask in a field of view of an imaging system of a        metrology tool, wherein a plurality of structures on the mask        are within the field of view of the imaging system;    -   b) measuring the coordinates of the plurality of structures on        the mask repeatedly with the metrology tool, wherein the mask is        shifted relative to the field of view of the imaging system        between the repeated measurements;    -   c) determining an alignment function for the imaging system from        the measurement results of step b, the alignment function being        a function of coordinates of the field of view of the imaging        system and giving an additive correction for optical errors in        the coordinates of the positions of structures measured by the        mask metrology tool;    -   d) correcting, for each structure of the plurality of structures        and for each shift of the mask relative to the field of view of        the imaging system, the measured position of the respective        structure in a mask coordinate system according to the alignment        function;    -   e) obtaining a final result for the position in the mask        coordinate system of each structure of the plurality of        structures by averaging over the corrected measured positions in        the mask coordinate system found in step d for the respective        structure at each relative position of mask and field of view of        the imaging system; and,    -   f) inferring mask writer errors from a comparison between the        final results for the positions obtained in step e and design        data for the mask measured.

According to the inventive method for correcting optical errors incoordinates of positions of targets measured via an imaging system,first an array of targets is placed within the field of view of theimaging system. It should be noted that in the context of thisapplication, “target” refers to any object or element the position ofwhich is to be measured via the mentioned imaging system. A target mayfor example be a structure on a surface of a semiconductor substrate oron a photolithography mask, but the inventive method is not limited tothese examples. The relative positions of the targets within the arrayof targets are fixed; or, put differently; with respect to a coordinatesystem relative to the array of targets the true values of thecoordinates of the targets are constant. It is important for the methodthat a plurality of targets of the array of targets is within the fieldof view.

Next, the array of targets is shifted to different relative positionswith respect to the field of view, and for each such relative positionthe coordinates of the plurality of targets are measured via the imagingsystem. Depending on the nature of the target, methods to measure thecoordinates of a target are known, which includes in particular also adefinition of what is to be understood as position of a target in thecase of spatially extended targets like lines on a wafer for example. Inembodiments the imaging system creates an image of the field of view ona detector, which, usually electronically, records the image and makesthe image available for evaluation, which in particular includesmeasuring positions of targets by known methods. The field of view andthe array of targets are two-dimensional, therefore the positions oftargets, o regardless whether relative to the field of view of relativeto the array of targets, are specified by two real numbers, typicallyreferred to as x- and y-coordinate of the respective target. The shiftsare also specified by two coordinates.

It is pointed out that as the relative shift between the array oftargets and the field of view is known, the relation between coordinatesrelative to the field of view and coordinates relative to a coordinatesystem of the array of targets is also known.

Then an alignment function is determined from the coordinates measuredin the preceding step, i.e. from the coordinates obtained for thetargets of the plurality of targets at each relative position of thearray of targets and the field of view. The alignment function is afunction of coordinates of the field of view of the imaging system andgives an additive correction for optical errors of the coordinates ofpositions of targets measured by the imaging system. This alignmentfunction is then used to correct the coordinates of the positions of thetargets measured by the imaging system. This is done by adding the valueof the alignment function at the respective field-of-view coordinates atwhich the coordinates of the position of each respective target weremeasured to the measured coordinates. In particular, in this waycorrected coordinate values relative to the array of targets can beobtained.

As the positions of the targets relative to the array of targets arefixed, any changes in the measured results of the coordinates oftargets, relative to the array of targets, are due to optical errorslike distortion or aberrations. In this way the inventive methodachieves a splitting of the sources of error, separating optical errorsfrom different types of error like placement errors of targetsintroduced when assembling the array of targets. The fact that aplurality of targets are in the field of view not only increases thethroughput, but also allows identifying changes in measured relativepositions of targets when comparing measurement results obtained atdifferent relative positions of the array of targets and the field ofview. Such changes can be attributed to optical errors, as positioningerrors of the array of targets when shifting the array of targets todifferent relative positions with respect to the field of view cannotaffect the relative positions of targets within the array.

As for any target of the plurality of targets corrected coordinatevalues relative to the array of targets are obtained by the methoddescribed so far for each relative position of the array of targets andthe field of view at which the coordinates of the respective target havebeen measured, in embodiments a final result for the coordinatesrelative to the array of targets of the respective target is determinedby averaging over the results obtained at the different relativepositions of the array of targets and the field of view for thecoordinates of the position relative to the array of targets of therespective target.

As has been said, for each relative position of the array of targets andthe field of view corrected values of the coordinates of its position,relative to the array of targets, can be obtained for each target. Inembodiments of the method the alignment function is determined in such away that the variation of these corrected values, for the entirety oftargets, is minimized. The reason for this approach is that in the idealcase, where there are no optical distortions or aberrations, the resultof the position of a specific target relative to the array of targetsshould be independent of the position in the field of view at which theformer position was measured, i.e. the position measurement relative tothe array of targets should not depend on the relative position of thearray of targets and the field of view. The variation for eachcoordinate is always considered a non-negative value, it may for examplebe the absolute value of the changes obtained, the square of thechanges, or the statistical variance. Then, for example, the variationsfor all the targets may be summed, or an average variation over thetargets may be used, for the step of minimizing the variation.

In specific embodiments the alignment function is determined byexpressing the alignment function as a linear combination of functions,wherein the coefficients of the linear combination are determined suchthat the aforementioned variation of coordinates relative to the arrayof targets is minimized. Typical functions used for building the linearcombination are polynomials up to a specified degree, cosine and/or sinefunctions, Chebyshev or Zernike polynomials. It is known to the skilledperson that the true or ideal alignment function, i.e. the alignmentfunction which would fully remove any error due to optical distortion oraberrations, is approximated by the linear combination the better themore functions are used therein, for example the higher the maximumdegree of the polynomials used or the higher the maximum order of thecosine/sine functions used is. Of course, the skilled person is notlimited to the examples of functions provided. Advantageously functionscan be used that are adapted to the expected symmetry properties of theoptical errors. Due to the two-dimensional nature of this fittingproblem, each term of the linear combination is to be regarded as afunction of two coordinates; such a function may, however, be written asa product of two functions of one coordinate each, of which, in specialcases, at least one may be a constant function.

In embodiments of the method the array of targets covers an area smallerthan the field of view of the imaging system. In such a case, all thetargets of the array may have their coordinates measured when performingthe method. In particular, the shifts of the relative position of thearray of targets and the field of view in embodiments are such that thetotal coverage of the area of the field of view achieved over the shiftsexceeds 85% of the area of the field of view. In specific embodiments,full coverage of the field of view, i.e. 100% coverage, is achieved. Theshifts used, and thus the relative positions between the array oftargets and the field of view, may be chosen such that the differentrequired relative positions can be assumed in a short time or accordingto any other criterion of import to a user of the method. An optimizedsequence of shifts, taking into account the geometry of the field ofview and the geometry of the array of targets may be determined prior tothe start of the position measurements by a pre-processing step.

In alternative embodiments the array of targets covers an area largerthan the field of view of the imaging system. Here, too, all the targetsin the array of targets may have their positions measured for performingthe method. A large number of targets usually provides better statisticsand a more reliable determination or better approximation of thealignment function. It is, however, also possible to use only a subsetof the targets of the array of targets for performing the method. If thearray of targets covers an area larger than the field of view, theplurality of targets having their positions measured at differentrelative positions of the array of targets and the field of view may notcomprise identical targets for each relative position. In order toprovide useful results, for each target the position of which ismeasured when carrying out the inventive method the position needs to bemeasured at more than one relative position of the array of targets andthe field of view.

In embodiments the array of targets is an arrangement of structures on asubstrate. Such a substrate may for example be a semiconductor water,and the structures are the structures created on the surface of thewafer by the known methods of water manufacturing. In an alternativeembodiment, the array of targets is given by the structures exhibited bya photolithography mask.

In a particularly advantageous embodiment, the array of targetsconstitutes a test pattern specifically designed for establishing thealignment function. Such a test pattern, in the case of structures on awafer, may for example be composed of holes or contacts which are at thelow end of the size scale of structures typically measured for suchwafers.

In embodiments, the measurement of the coordinates of the plurality oftargets for each relative position of the field of view and the array oftargets is repeated several times. This results in a higher number ofmeasurement values for the coordinates of the positions of the targetsthan if the measurement for the various relative positions of the fieldof view and the array of targets is only done once. Using this highernumber of measurement values in the determination of the alignmentfunction, where the alignment function is determined along the linesdescribed above, results in a higher precision of the resultingalignment function.

The inventive method for detecting photolithography mask writer errorsstarts by placing a mask in a field of view of an imaging system of ametrology tool, wherein a plurality of structures on the mask are withinthe field of view of the imaging system. The structures on the mask herecorrespond to the array of targets in the method described above.

Next, the coordinates of the plurality of structures on the mask aremeasured repeatedly with the metrology tool, wherein the mask is shiftedrelative to the field of view of the imaging system between the repeatedmeasurements. As in the method described above, the coordinates of theposition of a structure on the mask are given by two real numbers. Asthe shifts of the mask and the positions of the structures in the fieldof view are known after the respective measurement, the coordinates ofthe structures relative to the mask, i.e. in a mask coordinate system,are also known.

From the measurement results of the preceding step an alignment functionfor the imaging system is determined, the alignment function being afunction of coordinates of a field of view of the imaging system andgiving an additive correction for optical errors in the coordinates ofthe positions of structures measured by the metrology tool.

An alignment function, once obtained along the lines described above,can be stored for later use. Position measurements of targets can becorrected with a stored alignment function, where the stored alignmentfunction was not derived from measurements of the positions of thetargets that are to be corrected with it. Such position measurements canbe started when required, i.e. the full measurement of an array oftargets can be performed when needed, or coordinates of targets may bederived from images recorded with the imaging system at an earlier time,where such images have been stored for later use.

The alignment function is then used to correct, for each structure ofthe plurality of structures and for each shift of the mask relative tothe field of view of the imaging system, the measured position of therespective structure in a mask coordinate system.

A final result for the position in the mask coordinate system of eachrespective structure of the plurality of structures is then obtained byaveraging over the positions in the mask coordinate system found for therespective structure at each relative position of mask and field of viewof the imaging system.

As the optical errors introduced by the imaging system of the metrologytool are removed or at least suppressed by correcting the measuredcoordinate values with the alignment function, a comparison of thesecorrected values with the design data of the mask allows identifyingerrors due to the mask writer. In particular, a registration map for themask writer may be established, showing registration errors of the maskwriter over the surface of the mask.

As the structures on the mask are at fixed relative positions to eachother, any errors regarding the placement of structures on the maskduring the production of the mask are identical at each relativeposition of the mask and the field of view. Changes in the measuredpositions of the structures on the mask then are due to errors in theimaging system, i.e. optical errors like optical distortion and/oraberrations. By the inventive method it is therefore possible to removeor strongly suppress the optical error, without having to rely onassumptions about optical errors being low frequency and mask writererrors being high frequency errors. Especially, also low frequency maskwriter errors may be detected. Such low frequency mask writer errorsinclude, but are not limited to, rotation errors, magnification errorsand orthogonality errors.

As the positions of a plurality of structures in the field of view canbe measured simultaneously, the throughput is higher than in many priorart methods, with the additional advantage that the optical error, orfield-varying error as mentioned in the context of the prior artmulti-region-of-interest approach, is suppressed or removed. Thesimultaneous measurement of the positions of a plurality of targetstends to reduce the effect of noise on the measurement results, as thesimultaneous measurements of the plurality of targets is likely to beaffected in an equal manner by the noise, which implies an at leastpartial compensation of the noise effects, for example when relativepositions of structures are required. This leads to a reduction of thetotal measurement uncertainty.

In specific embodiments the mask exhibits mask writer qualificationpatterns, which may be holes, lines or further structures at the smallend of the size scale of structures occurring on the mask.

In analogy to the case of the method described before in the context ofgeneral arrays of targets, here the alignment function is determined insuch a way that the variation of the positions of the structuresrelative to the mask between the repeated measurements at differentrelative positions of the mask and the field of view is minimized whenthese positions are first corrected by the alignment function. Inspecific embodiments the alignment function is determined by expressingthe alignment function as a linear combination of functions, thecoefficients of the linear combination being determined in such a waythat the variation of the positions of the structures relative to themask between the repeated measurements is minimized when these positionsare first corrected by the alignment function. For further details onthe linear combination we refer to the above statements in the contextof the method referring to the general array of targets.

In embodiments, the plurality of structures covers an area smaller thanthe field of view of the imaging system. In particular embodiments theshifts of the mask are such that the total area covered by the pluralityof structures over the course of the shifts is more than 85% of the areaof the field of view. The coverage of the field of view may inparticular reach 100%.

In different embodiments the structures cover an area larger than thefield of view of the imaging system. Statements analogous to thepreviously described method referring to the array of targets apply.

BRIEF DESCRIPTION OF THE DRAWINGS

Below the invention and its advantages will be further described withreference to the accompanying schematic drawings. These drawings referspecifically to the case that the array of targets is a plurality ofstructures on a surface of a mask and the imaging system is the imagingsystem of a metrology tool. Nonetheless the figures can also illustratethe steps of the method referring to a more general array of targets.

FIG. 1 shows the schematic setup of a prior art metrology tool.

FIG. 2 shows the structures on the surface of a mask in relation to afield of view of an imaging system of a metrology tool.

FIG. 3 is analogous to FIG. 2, but for a translation of the maskrelative to the field of view.

FIG. 4 shows the areas in the field of view successively covered by themask over the course of successive shifts of the relative position ofthe mask and the field of view.

FIG. 5 shows successive relative shifts of a mask and the field of viewfor a case where the plurality of structures covers an area larger thanthe field of view.

FIG. 6 shows the flow diagram for a specific embodiment of the inventivemethod to determine mask writer errors.

DETAILED DESCRIPTION OF THE INVENTION

In the figures like reference numerals are used for like elements orelements of like function. Furthermore, for the sake of clarity, onlythose reference numerals are shown in the figures which are necessaryfor discussing the respective figure.

FIG. 1 shows a schematic representation of a coordinate measuringmachine or metrology tool 100 as has long been known from the prior art.The metrology tool 100 shown here is only one example of an apparatuswhich can be used to perform the methods according to the invention. Inno way are the methods according to the invention limited to thespecific configuration of such a metrology tool shown in the figure.What is important for the inventive methods is that an apparatus is ableto perform relative shifts between a mask and a field of view defined bythe apparatus, more precisely by an imaging system of the apparatus,that this field of view is such that a plurality of structures on themask is contained therein, and of course that the apparatus is able todetermine the positions of these structures.

A metrology tool 100 is used, for example, for determining the width(CD—critical dimension) of a structure 3 on a substrate 2. Also, usingthe metrology tool 100, the position of a structure 3 on the substrate 2can be determined. The substrate 2 may for example be a wafer with astructured surface or a mask exhibiting structures 3 to be transferredto a wafer by a photolithography process. Although the metrology tool100 shown in FIG. 1 has long been known from prior art, for the sake ofcompleteness, the operation of the metrology tool 100 and thearrangement of the individual elements of the metrology tool 100 will bediscussed.

The metrology tool 100 comprises a measuring table 20, which is arrangedso as to be displaceable on air bearings 21 in a plane 25 a, in theX-coordinate direction and in the Y-coordinate direction. For themounting of the measuring table 20, bearings other than air bearings canalso be used. The plane 25 a is formed from one element 25. In apreferred embodiment, the element 25 is granite. However, to a personskilled in the art, it is obvious that the element 25 can be made from adifferent material, which provides a precise plane for the displacementof the measuring table 20. The position of the measuring table 20 ismeasured by means of at least one laser interferometer 24 which, for themeasurement, emits a light beam 23 which hits the measuring table 20.From the position of the measuring table 20 the position of the mask 2can be inferred, in particular the position of the mask 2 relative to afield of view defined by an imaging system of the metrology tool 100.The element 25 itself is mounted on oscillation dampers 26 in order toprevent for example building oscillations reaching the device.

Placed on the measuring table 20 is a substrate 2, for example a mask,which bears the structures 3 to be measured. The substrate 2 can beilluminated with a transmitted light illumination apparatus 6 and/or areflected light illumination apparatus 14. The transmitted lightillumination apparatus 6 is provided in an optical arrangement 40. Thereflected light illumination apparatus 14 is also provided in an opticalarrangement 50. The optical arrangement 50 comprises the transmittedlight illumination apparatus, a deflecting mirror and a condenser. Bymeans of the deflecting mirror, the light from the transmitted lightillumination apparatus 6 is directed onto the condenser. The furtheroptical arrangement 50 comprises the reflected light illuminationapparatus 14, a beam-splitting mirror 12, the measuring objective 9 anda displacing device 15 assigned to the measuring objective 9. Using thedisplacing device 15, the measuring objective 9 can be displaced in theZ-coordinate direction (e.g. for focusing). The measuring objective 9collects light coming from the substrate 2, and the light is thendeflected out of the reflected light illumination axis 5 by means of thepartially transparent deflecting mirror 12. The light passes to a camera10 which is provided with a detector 11. The detector 11 is linked to acomputer system 16 which generates digital images from the measurementvalues determined by the detector 11.

The optical setup of the metrology tool, in particular the measuringobjective 9 and the camera 10 with detector 11, constitute an imagingsystem defining a field of view 1, as shown in FIGS. 2 and 3.

FIG. 2 shows a field of view 1 as defined by an imaging system of ametrology tool like that described in FIG. 1. A mask 2, exhibiting aplurality of structures 3 on its surface is placed in the field of view1. The structures may be of the same or different type, and arranged ina regular or irregular fashion. The plurality of structures 3 used inthe inventive methods may comprise all the structures on the mask 2 oronly a subset. In the figure, only the structures 3 used in the methodare shown schematically. With the field of view 1 there is associated afield-of-view coordinate system F with origin OF. With the mask 2 thereis associated a mask coordinate system 1, with origin OM.

The position of any structure 3 on the mask 2 can be specified relativeto the mask 2, i.e., in the mask coordinate system M, where its positionis given by two real numbers, Mx and My, say. The position of anystructure 3 can also be specified relative to the field of view 1, i.e.,in the field-of-view coordinate system F, where its position is given bytwo real numbers, Fx and Fy, say. Due to the mask placement and maskpositioning mechanisms, for example the measuring table 20 of themetrology tool 100 of FIG. 1, the relation between the mask coordinatesystem M and the field-of-view coordinate system F is known. This allowsexpressing the position of any structure 3 measured by a metrology toollike that in FIG. 1 either in field-of-view coordinates Fx, Fy or inmask coordinates Mx, My. The true positions of the structures 3 in themask coordinate system M are fixed, whereas the corresponding positionsin the field-of-view coordinate system F change of course, when the mask2 is shifted relative to the field of view 1.

Due to errors occurring in measurement procedures, the measured maskcoordinates Mx, My of a structure 3 may vary depending on circumstancesof the measurement. In particular, the measured mask coordinate valuesMx, My of a structure 3 will in general depend on the position in thefield of view 1 occupied by the structure 3 when its position ismeasured. When measuring the position of a structure 3, usually firstthe position of the structure in the field-of-view coordinate system Fis determined, as this position, expressed by coordinates to Fx and Fy,is the quantity directly accessible via, for example, the image of thefield of view 1 recorded by a detector 11 of the mask metrology tool100, as shown in FIG. 1. From the known relation between the maskcoordinate system M and the field-of-view coordinate system F, themeasured values Mx, My of the coordinates of the structure in the maskcoordinate system NI can be derived from the field-of-view coordinatesFx and Fy.

FIG. 3 shows a similar situation as in FIG. 2; most elements shown inFIG. 3 have already been described in FIG. 2. In FIG. 3 the mask 2 hasbeen shifted relative to the field of view 1, in comparison with FIG. 2.The shift is expressed by a translation T of the origin OM of the maskcoordinate system M relative to the origin OF of the field-of-viewcoordinate system F.

According to the methods of the invention, the mask 2 is translated todifferent positions relative to the field of view 1. In each suchrelative position the coordinates of the structures 3 in the maskcoordinate system M are determined by measurements, as described above.As has also been said above, the true coordinates of the structures 3 inthe mask coordinate system M are independent of the position of the mask2 relative to the field of view 1. Therefore an alignment functioncorrecting the optical errors, which depend on the position in the fieldof view 1 where a measurement is done, is constructed by steps,described above, which minimize the variation in the measured maskcoordinate values Mx and My obtained for one and the same structure 3when the positions of the structures 3 are measured at differentrelative positions of the mask 2 and the field of view 1.

FIG. 4 illustrates how, by successive relative shifts, indicated by thearrows, of the mask 2, the field of view 1 is covered to a substantialdegree. A high percentage of coverage of the field of view 1 isnecessary to determine the alignment function to sufficient precisionfor the entire field of view 1. The sequence of successive shifts may bechosen before the start of a measurement, taking into account thegeometry of the field of view 1 and of the mask 2. It is also possiblethat in the sequence of such shifts the areas successively occupied bythe mask 2 partially overlap. In this way a coverage of the area of thefield of view up to 100% can be achieved. For the sake of clarity,structures 3 are not shown on the mask 2 in this figure.

FIG. 5 illustrates a case where the plurality of structures 3 thepositions of to which are measured in the inventive methods cover anarea larger than the field of view 1. The field of view 1 is showntwice, once in solid line, once in dashed line. These tworepresentations of the field of view 1 correspond to two differentrelative positions of the field of view 1 and the mask 2, successivelyassumed according to the inventive methods. The field of view 1 is shownonly for two such relative positions, for the sake of clarity; it is ofcourse possible to cover the entire mask 2 in this way. This will benecessary, if mask writer errors for the entire mask are to beidentified, for example. The different relative positions of the mask 2and the field of view 1 here have to be chosen such that at least forthe structures 3 to be used in the method the positions of thesestructures 3 relative to the mask 2 are measured at more than onerelative position of the mask 2 and the field of view 1. This requiresthat, if the field of view 1 is shown on the mask 2 at the correspondingdifferent relative positions with respect to the mask 2, there is anoverlap of the representations of the field of view 1, as is indicatedhere.

FIG. 6 shows the flow chart for a particular implementation of theinventive method for identifying mask writer errors. In step 200, aplurality of structures 3 on mask 2 is selected; for this example, theplurality of structures shall comprise a number N of structures. Thepositions of these structures will be measured repeatedly in the method.

In step 202, one position of the mask 2 relative to the field of view 1and the N structures to be used are prepared as a job definition for ametrology tool 100. The job definition in this example defines therelative positions of the mask and the field of view in terms of a siteon the mask which is to he centered in the field of view. The jobdefinition is, in step 204, passed through a pre-processor, which may beimplemented in the metrology tool or on external hardware, usually assoftware. The pre-processor creates a modified job definition,specifying the N structures to be measured, and a number S of sites onthe mask 2 to be successively centered in the field of view 1 of themetrology tool 100. Specifying this number S of sites corresponds tospecifying a sequence of shills of the mask 2 relative to the field ofview 1. The modified job definition is output in step 206.

In step 208 the modified job definition is carried out by the metrologytool 100. The results, in step 210, in measurement results for thecoordinates of the N structures measured at each of the S relativepositions of the field of view 1 and the mask 2. These measurementresults are subject to post-processing in step 212. The post-processingcan be implemented in the metrology tool 100 or on external hardware. Inany case, it is usually implemented in software. The post-processingstep 212 determines an alignment function according to the correspondingsteps of the method described above, and uses the alignment function tocorrect the measurement results provided in step 210 for optical errorslike distortion and/or aberrations. The results, in step 214, in Ncorrected positions of structures, each position being specified by twocoordinates in the mask coordinate system.

These corrected position values, in step 216, are input to an evaluationprocess, which determines, for each structure, the deviation between thecorrected position, in mask coordinates, of the structure and an idealposition, provided for example as design data of the mask. Thedeviations obtained this way for the entirety of the N structures thenresults in a local registration map for the mask 2 in step 218. Thelocal registration map shows the errors of the mask writer used toproduce the mask 2 used in the process just described.

The invention has been described with reference to preferredembodiments. It is, however, known to the skilled person thatalterations and modifications are possible without leaving the scope ofthe subsequent claims.

What is claimed is:
 1. A method for correcting optical errors occurringin coordinates of positions of targets measured via an imaging system,the method comprising the following steps: a) placing an array oftargets in a field of view of the imaging system, wherein a plurality oftargets of the array of targets are within the field of view of theimaging system and wherein the relative positions of targets within thearray of targets are fixed; b) measuring the coordinates of theplurality of targets of the array of targets repeatedly via the imagingsystem, wherein the array of targets is shifted relative to the field ofview of the imaging system between the repeated measurements; c)determining an alignment function from the measurement results of stepb, the alignment function being a function of coordinates of the fieldof view of the imaging system and giving an additive correction foroptical errors of the coordinates of positions of targets measured bythe imaging system; and d) correcting the coordinates of the positionsof the targets measured by the imaging system by adding the respectivevalue of the alignment function at the field-of-view coordinates atwhich the coordinates of the position of the respective target weremeasured.
 2. The method of claim 1, wherein a final result for thecoordinates of a position relative to the array of targets of arespective target is determined by averaging over the results obtainedat different relative positions of the array of targets and the field ofview for the coordinates of the position relative to the array oftargets of the respective target, corrected by the alignment function.3. The method of claim I, wherein the alignment function is determinedin step c such that a variation of the measured positions of targetsrelative to the array of targets between the repeated measurements isminimized when these measured positions are first corrected by thealignment function.
 4. The method of claim 3, wherein the alignmentfunction is determined by expressing the alignment function as a linearcombination of functions, the coefficients of the linear combinationbeing determined such that the variation of the measured positions oftargets relative to the array of targets between the repeatedmeasurements is minimized when these measured positions are firstcorrected by the alignment function.
 5. The method of claim 1, whereinthe array of targets covers an area smaller than the field. of view ofthe imaging system.
 6. The method of claim 5, wherein the shifts of thearray of targets are such that the total area covered by the array oftargets over the course of the shifts is more than 85% of the area ofthe field of view.
 7. The method of claim 1, wherein the array oftargets covers an area larger than the field of view of the imagingsystem.
 8. The method of claim 1, wherein the array of targets is anarrangement of structures on a substrate.
 9. The method of claim 8,wherein the array of targets is a structured semiconductor wafer. 10.The method of claim 8, wherein the array of targets is aphotolithography mask.
 11. The method of claim 1, wherein the array oftargets constitutes a specifically designed test pattern.
 12. The methodof claim 1, wherein step b is repeated several times, and all resultingmeasured coordinates are used for determining the alignment function instep c.
 13. The method of claim 1, wherein after step c, the alignmentfunction is stored, and the stored alignment function is applied tocorrect coordinates of positions of targets measured via the imagingsystem at a later time, where the correction is achieved by adding therespective value of the alignment function at the field-of-viewcoordinates at which the coordinates of the position of the respectivetarget were measured.
 14. A method for detecting photolithography maskwriter errors, comprising the following steps: a) placing a mask in afield of view of an imaging system of a metrology tool, wherein aplurality of structures on the mask are within the field of view of theimaging system; b) measuring the coordinates of the plurality ofstructures on the mask repeatedly with the metrology tool, wherein themask is shifted relative to the field of view of the imaging systembetween the repeated measurements; c) determining an alignment functionfor the imaging system from the measurement results of step b, thealignment function being a function of coordinates of the field of viewof the imaging system and giving an additive correction for opticalerrors in the coordinates of the positions of structures measured by themetrology tool; d) correcting, for each structure of the plurality ofstructures and for each shift of the mask relative to the field of viewof the imaging system, the measured position of the respective structurein a mask coordinate system according to the alignment function; e)obtaining a final result for the position in the mask coordinate systemof each structure of the plurality of structures by averaging over thecorrected measured positions in the mask coordinate system found in stepd for the respective structure at each relative position of mask andfield of view of the imaging system; and f) inferring mask writer errorsfrom a comparison between the final results for the positions obtainedin step e and design data for the mask measured.
 15. The method of claim14, wherein the mask exhibits mask writer qualification patterns. 16.The method of claim 14, wherein the alignment function is determined instep c such that a variation of the positions of the structures relativeto the mask between the repeated measurements is minimized when thesemeasured positions are first corrected by the alignment function. 17.The method of claim 16, wherein the alignment function is determined byexpressing the alignment function as a linear combination of functions,the coefficients of the linear combination being determined such thatthe variation of the positions of the structures relative to the maskbetween the repeated measurements is minimized when these measuredpositions are first corrected by the alignment function.
 18. The methodof claim 14, wherein the plurality of structures covers an area smallerthan the field of view of the imaging system.
 19. The method of claim18, wherein the shifts of the mask are such that the total area coveredby the plurality of structures over the course of the shifts is morethan 85% of the area of the field of view.
 20. The method of claim 14,wherein the plurality of structures covers an area larger than the fieldof view of the imaging system.
 21. The method of claim 14, wherein stepb is repeated several times, and all resulting measured coordinates areused for determining the alignment function in step c.
 22. A method fordetecting photolithography mask writer errors, comprising the followingsteps: a) placing a mask exhibiting structures which constitute a maskwriter qualification pattern in a field of view of an imaging system ofa metrology tool, wherein a plurality of the structures on the mask arewithin the field of view of the imaging system; b) measuring thecoordinates of the plurality of structures on the mask repeatedly withthe metrology tool, wherein the mask is shifted relative to the field ofview of the imaging system between the repeated measurements; c)determining an alignment function for the imaging system from themeasurement results of step b, the alignment function being a functionof coordinates of the field of view of the imaging system and giving anadditive correction for optical errors in the coordinates of thepositions of structures measured by the metrology tool, the alignmentfunction being determined by expressing the alignment function as alinear combination of functions and determining the coefficients of thelinear combination such that a variation of the positions of thestructures in a mask coordinate system between the repeated measurementsis minimized when these measured positions are first corrected by thealignment function; d) correcting, for each structure of the pluralityof structures and for each shift of the mask relative to the field ofview of the imaging system, the measured position of the structure inthe mask coordinate system according to the alignment function; e)obtaining a final result for the position in the mask coordinate systemof each structure of the plurality of structures by averaging over thecorrected positions in the mask coordinate system found for therespective structure at each relative position of mask and field of viewof the imaging system used in step d; and f) inferring mask writererrors from a comparison between the final results for the positionsobtained in step e and design data for the mask measured.
 23. The methodof claim 22, wherein after step c, the alignment function is stored, andthe stored alignment function is applied to correct coordinates ofpositions of structures in the mark coordinate system at a later time,where the correction is achieved by adding the respective value of thealignment function at the field-of-view coordinates at which thecoordinates of the position of the respective structure were measured.